Two major developments have emerged in signal system design over the last 20 years. One is a microprocessor based local controller that utilizes software that is flexible enough to deal with phase skipping and overlapping using the dual-ring concept. The dual-ring concept is employed, for example, in so-called California 170 local controllers and in NEMA specification local controllers. The other is the development of relatively small area-wide control systems that use a personal computer as the central computer and software developed for the general purpose computer market, such as communication and database management software. Traffic control systems which include computers are shown, for example, in U.S. Pat. Nos. 3,363,185, Sanderson et al; 3,764,972, Siklos et al; 3,828,307, Hungerford; and 3,886,496, Spilo et al.
Prior art signal control systems are not designed to be self-adaptive to ever-changing traffic demands within the system boundary. They require signal parameter update every several years to maintain proper signal timing. With prior art systems human involvement is required in timing plan preparation, which work is performed at the control center under control of the traffic engineer. With the present traffic control system the local controller functions to control signals with minimal interface with the central system.
The new local controller is smart enough (i.e. has sufficient memory and fast enough CPUs) to automatically compute (or adjust) the proper signal timing for the prevailing traffic conditions, not only for the local intersection, but also for the areawide system. The new local controller is designed to be flexible enough to easily accommodate various traffic signal control (or optimization) methods.
The signal indication control (signal switching) software is flexible enough to handle practically all intersection geometries and easily incorporate various signal phasing and timing control algorithms.
In addition, the new local controller is designed to be flexible enough to accommodate most, if not all, traffic signal control (or optimization) methods that will be developed after the system is installed and becomes operational.
Features of the new local controller include the following:
a) The CPU that handles signal control algorithm processing is fast enough to handle most conceivable signal control algorithms.
b) Transmission between the local controller and the local master can encompass all the data types for most conceivable algorithms, and the transmission capacity is large enough to handle these data types.
c) The signal switching mechanism, especially the phasing method, is flexible enough to execute practically all conceivable algorithms.
d) The software design is modular so that any signal control algorithm may be easily plugged into the advanced algorithm processor (AAP).
e) The software design is flexible enough to switch from operating one algorithm to another.
f) The signal control software is able to automatically compute (or adjust) the proper signal timing for the prevailing traffic, not only from the local intersection control standpoint, but also from the area-wide signal control standpoint (if it is not for isolated intersection control).
g) The on-line real-time traffic-responsive and traffic-adaptive signal control algorithm for signal timing reflects the prevailing traffic conditions on an on-line real-time basis and maintains coordination with neighboring intersection signals.